Aftermath of the Permian – Triassic Mass Extinction

A new study of fossil fishes from Middle Triassic sediments on the shores of Lake Lugano provides new insights into the recovery of biodiversity following the great mass extinction event at the Permian-Triassic boundary 240 million years ago.

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The largest episode of mass extinction in the history of the Earth, which led to the demise of about 90% of marine organisms and a majority of terrestrial species, took place between the Late Permian and Early Triassic, around 240 million years ago. How long it took for biological communities to recover from such a catastrophic loss of biodiversity remains the subject of controversial debate among paleontologists.

A new study of fossil fishes from Middle Triassic strata on the shores of Lake Lugano throws new light on the issue. The study, undertaken by researchers led by Dr. Adriana López-Arbarello, who is a member of the GeoBiocenter at Ludwig-Maximilians-Universitaet (LMU) in Munich and the Bavarian State Collection for Paleontology and Geology, suggests that the process of recovery was well underway within a few million years. The authors, including Dr. Heinz Furrer of Zurich University and Dr. Rudolf Stockar of the Museo Cantonale di Storia Naturale in Lugano, who led the excavations at the sites, and Dr. Toni Bürgin of the Naturmuseum St. Gallen report their findings in the journal PeerJ.

The fossil fishes analyzed by López-Arbarello and her colleagues originate from Monte San Giorgio in the canton Ticino in Switzerland, which is one of the most important sources of marine fossils from the Middle Triassic in the world. The Monte San Giorgio rises to an altitude of 1000 m on the promontory that separates the southern arms of Lake Lugano in the Southern Swiss Alps. But in the Middle Triassic, it was part of a shallow basin dotted with islands fringed by lagoons, which were separated by reefs from the open sea. “The particular significance of its fossil fauna lies in the careful stratigraphic work that has accompanied the excavations here.

The positions of each of the fossil finds discovered here have been documented to the centimeter,” says Adriana López-Arbarello. On the basis of detailed anatomical studies of new material and a taxonomic re-evaluation of previously known specimens from the locality, she and her colleagues have identified a new genus of fossil neopterygians, which they name Ticinolepis. The Neopterygii include the teleost fishes, which account for more than half of all extant vertebrate species. However, the new fossil species are assigned to the second major group of neopterygians, the Holostei, of which only a handful of species survives today. The researchers assign two new fossil species to the genus Ticinolepis, namely T. longaeva and T. crassidens, which occur in different sedimentary beds within the so-called Besano Formation on Monte San Giorgio.

The two species coexisted side by side but they occupied distinct ecological niches. T. crassidens fed on mollusks and was equipped with jaws and teeth that could handle their hard calcareous shells. T. longaeva was more of a generalist, and was found in waters in which T. crassidens could not survive. The authors interpret the different distribution patterns as a reflection of changing environmental conditions following the preceding mass extinction event.

The less specialized T. longaeva was able to exploit a broader range of food items, and could thus adapt more flexibly to fluctuating conditions. On the other hand, the dietary differentiation between the two species indicates that a variety of well-established ecosystems was available in the Besano Formation at this time. “This in turn suggests that the marine biota is likely to have recovered from the great mass extinction relatively quickly,” Adriana López-Arbarello concludes.

New Study Proposes Short and Long Process of Extinction

A new study of nearly 22,000 fossils finds that ancient plankton communities began changing in important ways as much as 400,000 years before massive die-offs ensued during the first of Earth’s five great extinctions.

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The research, published July 18 in the Early Edition of the Proceedings of the National Academy of Sciences, focused on large zooplankton called graptolites. It suggests that the effects of environmental degradation can be subtle until they reach a tipping point, at which dramatic declines in population begins.

“In looking at these organisms, what we saw was a disruption of community structures – the way in which the plankton were organized in the water column. Communities came to be less complex and dominated by fewer species well before the massive extinction itself,” says co-author H. David Sheets, PhD, professor of physics at Canisius College and associate research professor in the Evolution, Ecology and Behavior graduate program at the University at Buffalo.

This turmoil, occurring in a time of ancient climate change, could hold lessons for the modern world, says co-author Charles E. Mitchell, PhD, professor of geology in the University at Buffalo College of Arts and Sciences.

The shifts took place at the end of the Ordovician Period some 450 million years ago as the planet transitioned from a warm era into a cooler one, leading eventually to glaciation and lower sea levels.

“Our research suggests that ecosystems often respond in stepwise and mostly predictable ways to changes in the physical environment – until they can’t. Then we see much larger, more abrupt, and ecologically disruptive changes,” Mitchell says. “The nature of such tipping point effects are hard to foresee and, at least in this case, they led to large and permanent changes in the composition of the oceans’ living communities.

“I think we need to be quite concerned about where our current ocean communities may be headed or we may find ourselves at the tail end of a similar event – a sixth mass extinction, living in a very different world than we would like.” The study was a partnership between Canisius, UB, St. Francis Xavier University, Dalhousie University and The Czech Academy of Sciences.

A long slide toward oblivion

In considering mass extinction, there is perhaps the temptation to think of such events as rapid and sudden: At one moment in history, various species are present, and the next they are not.

This might be the conclusion you’d draw if you examined only whether different species of graptolites were present in the fossil record in the years immediately preceding and following the Ordovician extinction.

“If you just looked at whether they were present – if they were there or not – they were there right up to the brink of the extinction,” Sheets says. “But in reality, these communities had begun declining quite a while before species started going extinct.”

The research teased out these details by using 21,946 fossil specimens from areas of Nevada in the U.S. and the Yukon in Canada that were once ancient sea beds to paint a picture of graptolite evolution.

The analysis found that as ocean circulation patterns began to shift hundreds of thousands of years before the Ordovician extinction, graptolite communities that previously included a rich array of both shallow- and deep-sea species began to lose their diversity and complexity.

Deep-water graptolites became progressively rarer in comparison to their shallow-water counterparts, which came to dominate the ocean.

“There was less variety of organisms, and the rare organisms got rarer,” Sheets says. “In the aftermath of a forest fire in the modern world, you might find that there are fewer organisms left – that the ecosystem just doesn’t have the same structure and richness as before. That’s the same pattern we see here.”

The dwindling deep-sea graptolites were species that specialized in obtaining nutrients from low-oxygen zones of the ocean. A decrease in the availability of such habitats may have sparked the creatures’ decline, Sheets and Mitchell say.

“Temperature changes drive deep ocean circulations, and we think the deep-water graptolites lost their habitats as the climate changed,” Sheets says. “As the nature of the oceans shifted, their way of life went away.”

SPECIAL REPORT: Study Shows Weakened Magnetic Field Has No Effect on Avian Compass

Reporting their results in the New Journal of Physics, scientists have taken a step forward in unraveling the inner workings of the avian compass – a puzzle that has captivated researchers for decades. The team, led by a group at Oxford University, is exploring the possibilities of a weakened Earth’s magnetic field would have on living organisms.

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Magnetic sensing is a type of sensory perception that has long been studied. Over the past 50 years, scientific studies have shown a wide variety of living organisms have the ability to perceive magnetic fields and can use information from the Earth’s magnetic field in orientation behavior. Examples abound: salmon, sea turtles, spotted newts, lobsters, honeybees, and perhaps us humans, most of which can perceive and utilize geomagnetic field information.

The avian magnetic compass is a complex entity with many surprising properties. The basis for the magnetic sense is located in the eye of the creature, and furthermore, it is light-dependent. The most accepted theory is living organisms or themselves via magnetically sensitive chemical reactions, which take place in proteins known as cryptochromes present in the eyes retina.

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Scientific studies have confirmed that humans do in fact have both magnetite and cryptochromes hardwired as part of our biological makeup. Using an ultrasensitive superconducting magnetometer in a clean-lab environment, scientists have detected the presence of ferromagnetic material in a variety of tissues from the human brain. Magnetic particle extracts from solubilized brain tissues examined with high-resolution transmission electron microscopy, electron diffraction, and elemental analyses identify minerals in the magnetite-maghemite family.

Now the question is, does the weakening Earth’s magnetic field have an effect on living organisms? “The principle that chemical transformations can respond to very weak magnetic fields, known as the radical pair mechanism, is unquestionably genuine,” said Peter Hore, a biophysical chemist at Oxford University, who is heading up the study. “What is not yet proven is whether this mechanism lies at the heart of avian magneto-reception (The ability to perceive magnetic fields).”

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According to Hore, probably the most serious stumbling block is whether the spin coherence in the radicals (the short-lived chemical intermediates responsible for the magnetic field effect) could last long enough to allow a magnetic field as weak as the Earth’s to alter the photochemistry of a cryptochrome.

To find out more, the team has built a computational model focusing on the internal magnetic interactions within and between the radicals involved in the process. The simulations allow the scientists to examine the modulation of these interactions caused by thermal fluctuations in the positions of the radicals in their binding sites in the cryptochrome.

Examining the data, the group observes the effect of a weakening Earth magnetic field is sufficient to change the proportion of radical pairs that proceed along two competing chemical reaction pathways. “The effect happens in such a way that the yield of the signaling state of  protein should depend on the direction of the magnetic field with respect to the cryptochrome molecule,” Hore adds. “Furthermore, our results show the loss of coherence caused by certain sorts of internal magnetic interactions and molecular dynamics could actually enhance, rather than degrade, the sensitivity of a cryptochrome-based magnetic compass sensor.”

Device applications
Thinking further ahead, the researchers highlight that their findings could benefit the development of low-cost and more environmentally-friendly electronic devices. “Certain organic semiconductors (OLEDs, for example) exhibit magneto-electro-luminescence or magneto-conductance, the mechanism of which shares essentially identical physics with radical pairs,” said Hore. “I believe there is scope for the design and construction of electronically addressable devices, based on principles learnt from studies of the avian compass, for determining the presence, intensity and direction of weak magnetic fields using cheap, non-toxic organic materials.”

 

Cryptochrome and Magnetic Sensing

Magnetic sensing is a type of sensory perception that has long been studied. Over the past 50 years, scientific studies have shown that a wide variety of living organisms have the ability to perceive magnetic fields and can use information from the Earth’s magnetic field in orientation behavior. Examples abound: salmon, sea turtles, spotted newts, lobsters, honeybees and perhaps us humans can all perceive and utilize geomagnetic field information.

cryptochrome

But perhaps the most well-studied example of animal magnetoreception is the case of migratory birds (e.g. European robins (Erithacus rubecula), silvereyes (Zosterops l. lateralis), garden warblers (Sylvia borin)), who use the Earth’s magnetic field, as well as a variety of other environmental cues, to find their way during migration.

The avian magnetic compass is a complex entity with many surprising properties. The basis for the magnetic sense is located in the eye of the bird, and furthermore, it is light-dependent, i.e., a bird can only sense the magnetic field if certain wavelengths of light are available. Specifically, many studies have shown that birds can only orient if blue light is present. The avian compass is also an inclination-only compass, meaning that it can sense changes in the inclination of magnetic field lines but is not sensitive to the polarity of the field lines. Under normal conditions, birds are sensitive to only a narrow band of magnetic field strengths around the geomagnetic field strength, but can orient at higher or lower magnetic field strengths given accommodation time.

A Radical-Pair-Based Avian Compass

Despite decades of study, the physical basis of the avian magnetic sense remains elusive. The two main models for avian magnetoreception are a magnetite-based model and a radical-pair-based model (for review see, e.g., Solov’yov, Schulten, Greiner, 2010). The former suggests that the compass has its foundation in small particles of magnetite located in the head of the bird. The latter idea is that the avian compass may be produced in a chemical reaction in the eye of the bird, involving the production of a radical pair. A radical pair, most generally, is a pair of molecules, each of which have an unpaired electron. If the radical pair is formed so that the spins on the two unpaired electrons in the system are entangled (i.e. they begin in a singlet or triplet state), and the reaction products are spin-dependent (i.e., there are distinct products for the cases where the radical pair system is in an overall singlet vs. triplet state), then there is an opportunity for an external magnetic field to affect the reaction by modulating the relative orientation of the electron spins.

How could a radical pair reaction lead to a magnetic compass sense? Suppose that the products of a radical pair reaction in the retina of a bird could in some way affect the sensitivity of light receptors in the eye, so that modulation of the reaction products by a magnetic field would lead to modulation of the bird’s visual sense, producing brighter or darker regions in the bird’s field of view. (The last supposition must be understood to be speculative; the particular way in which the radical pair mechanism interfaces with the bird’s perception is not well understood.)

When the bird moves its head, changing the angle between its head and the Earth’s magnetic field, the pattern of dark spots would move across its field of vision and it could use that pattern to orient itself with respect to the magnetic field. This idea is explored in detail by Ritz et al (see below). Interestingly, studies have shown that migratory birds exhibit a head-scanning behavior when using the magnetic field to orient that would be consistent with such a picture. Such a vision-based radical-pair-based model would explain several of the unique characteristics of the avian compass, e.g., that it is light-dependent, inclination-only, and linked with the eye of the bird. It is also consistent with experiments involving the effects of low-intensity radio frequency radiation on bird orientation, as suggested by Canfield et al.

The question remains as to where, physically, this radical pair reaction would take place. It has been suggested that the radical pair reaction linked to the avian compass arises in the protein cryptochrome. Cryptochrome is a signaling protein found in a wide variety of plants and animals, and is highly homologous to DNA photolyase. There is some evidence that retinal cryptochromes may be involved in the avian magnetic sense. Detailed analysis of cryptochrome as a transducer for the avian compass would require an atomic-resolution structure of the protein, and unfortunately, no structure of avian cryptochrome is currently available.

However, the structure of cryptochrome from a plant (Arabidopsis thaliana) is available, and the cryptochromes of plants and birds are structurally very similar. Recent experiments by Ahmad et al. (Ahmad, Galland, Ritz, Wiltschko and Wiltschko. Magnetic intensity affects cryptochrome-dependent responses in Arabidopsis thaliana. Planta 225, 615-624 (2007)) have shown that Arabidopsis seedlings exhibit a magnetic field effect. Processes involved with cryptochrome signaling (such as hypocotyl growth inhibition) are enhanced under a magnetic field of 5 G (as compared with an Earth-strength 0.5 G magnetic field).

Both photolyase and cryptochrome internally bind the chromophore flavin adenine dinucleotide (FAD). In photolyase, the protein is brought to its active state via a light-induced photoreduction pathway involving a chain of three tryptophans. Studies suggest that cryptochrome also is activated by a similar photoreduction pathway.

However cryptochrome’s signalling state has a limited lifetime. Under aerobic conditions, the stable FADH molecule slowly reverts back to the initial FAD state as illustrated in Fig. 3. This process is not well understood and occurs on the millisecond time scale. The cryptochrome back-reaction attracted considerable attention recently due to indications that it may be the key link to avian magnetoreception. In the course of the back-reaction a radical pair is formed between flavin and an oxygen molecule.